Frequency modulation radio transmitter



Nov. 29, 1949 J. c. GEIST FREQUENCY MODULATION RADIO TRANSMITTER FiledNOV. 13, 1945 om, n

INVENTOR. JOH N C. GEIST Patented Nov. 29, 1949 FREQUENCY MODULATIONRADI@ TRANSMITTER,

John C. Geist, Avon, N. .it Application November 13, 1945, Serial No.628,342

(Cl. 33t-Ml 6 Claims..

(Granted under the act oi amended April 3m 1928;'

The invention described herein may be manu factured and used by or forthe Government for governmental purposes, without the payment to me ofany royalty thereon.

The present invention relates to radio transmitter systems. Moreparticularly, the present invention relates to frequency modulationtransmitter systems, and has for its primary object to provide asimplied and low cost system of the character referred to for generalradio signal communication.

It has been customary to use amplitude "modwl lation for radio telephonetransmission 1n the minor or low power classes of stations asdistinguished from commercial and high fidelity stations using higherpower and costlier equipment and in more complicated circuit networks.Furthermore, frequency modulation networks heretofore used for broadcastsignals have had the disadvantage of requiring specially designedcomponents and complex circuits requiring adjustment to a high degree oftolerance and are relatively expensive.

It is also an object oi this invention to provide a new and improvedfrequency modulation system for radio signal transmission that is lesscoin-u plicated, and limited in its requirements for circuit componentsand operational adjustments.

It is a further object of the invention to provide a new and improvedfrequency modulation sys-n tem or network for general radio signaltransmission that is simple in structural requirements, is of amplepractical capacity and avoids the customary critical requirements invalues and types of equipment considered essential in conventionalnetworks.

A still further object of the present invention is to provide a new andimproved frequency modulation network for minor radio telephonetransmission that will retain all the general inherent advantages offrequency modulation signal transmission without involving its customarycomplexity and cost.

For a better understanding of the invention and further objects thereof,reference is made to the accompanying drawings and the followingdescription while the scope of the invention is particularly pointed outin the appended claims.

Referring to the drawings:

Figure 1 is a schematic circuit diagram of a frequency modulationtransmitter system embodying the invention, and

Figure 2 is a View in perspective of a signal transmitter deviceconstructed in accordance March 5i. 1889, as

37d (l). Gi. 75th with the invention to include the system ci? Figure i.

Referring to Figure i, the transmitter system may be considered tocomprise six main sections, as follows: A modulator an oscillator t, aprio mary multiplier amplifier t, an auxiliary multi plier amplier t, afinal stage amplier i, and a radiator device or antenna t.

The oscillator is preferably oi' a type shown, comprising an oscillatortube ill having a tuned tank circuit Ii connected between the anode itand the control grid it. The tank circuit is con nected directly withthe anode and is coupled to the control grid through a suitable anodecurrent blocking capacitor iii. The grid circuit is completed to groundand cathode through an R. F. choke coil i5 and a grid bias resistor I5.The tank circuit for the oscillator comprises a tuning inductance Il!shunted by a split-stator tuning capacitor lt-i@.

The tuning inductance il is provided with tap 2d through which anodecurrent is applied to the tube It from an anode current supply circuitZi-Zt connected with a positive supply terminati 2t. interposed betweenthe supply lead ti and the tap 20 is an R. F. choke coil it and anaudio= frequency or modulation choke coil 25 in series, the R. F. chokecoil being connected more ad jacent to the tap 2li on the oscillatortuning inductance.

in a transmitter designed to operate in the Vfrequency range between ll2and 115.5 mega= cycles. a commercial type 705 tube at It has been foundto be satisfactory, with the screen grid 2t connected to the anode, andhas been. operated as a self-excited ultra-audion oscillator on afrequency of 56.5 megacycles. The anode voltage applied to the tube wasof the order of 18d volts with an anode current oi approximately 28milli-amperes. 'I'he voltage provided at the positive terminal 23 may belowered to the proper voltage for the anode through a suitable seriesresistor indicated at 2l in the anode current supply circuit lead 2i.

'I'he oscillator is amplitude modulated at a re1atively low percentagemodulation by a suitable amplitude modulation means rei'erred togenerally in the unit 3 and comprising in the present example, amicrophone 30 representing any suitable source of modulation signalscoupled through a microphone transformer 3l and a modulation gaincontrol potentiometer 32 with a self-biased amplifier tube 33. Theoutput or anode circuit 34 of the tube 33 is coupled through an outputlead 35 and a coupling capacitor 36 with a point 31 on the anode circuitof the oscillator located between the R. F. choke 24 and theaudiofrequency choke 25, thus introducing into the oscillator anodecircuit the modulating frequency from the amplifier 33 resulting fromspeech or other audio-frequency sound being applied to the microphone 38in the usual manner.

The audio-frequency output of the amplifier tube 33 is derived from anaudio-frequency choke coil 38 connected in the anode circuit 34 betweenthe anode 39 and the positive anode current supply terminal 23. A commonreturn supply terminal 40 is connected to ground and to the cathodes ofthe oscillator and the amplifier as shown. A suitable battery 4|connected between terminals 23 and 48 is indicated as a source of anodecurrent, although any other suitable direct current source of supply maybe used.

Any suitable means may be provided for amplitude modulating theoscillator. In the present example, the output from the modulator isapplied to the oscillator through the terminal 31 between the oscillatortuning inductance |1 and the modulation choke coil 25, whereby theeffective anode voltage of the oscillator is varied in accordance withthe modulation frequency. Amplitude modulation of the self-excitedoscillator causes frequency modulation of the oscillator, thatisfvariation of the frequency existing in the tank circuit To providefrequency multiplying and to remove from the output of the oscillatorthe amplitude modulation component of the resulting signal, theoscillator is coupled to the multiplying amplifier which is of the classC type and comprises an amplifier tube 45, which is biased beyondcutoff, a tuned input circuit 46, and a tuned output circuit 41. Asuitable source of bias potential is indicated at ||4. The tuned inputcircuit includes a tuning inductance 4s provided with a shunt tuningcapacitor 49, the inductance being coupled to the tuning inductance ofthe oscillator, as shown, for transferring energy therefrom. Thefrequency of this circuit is the same as that of the oscillator, whilethe output circuit 41 is provided with a tuning inductance 58 and ashunt tuning capacitor 5| to provide tuning to a multiple of theoscillator frequency such as, in the present example, 113 megacycles.The amplifier tube 45 may be of the commercial type 105, operating, asdescribed, as a class C doubler inductively coupled to the oscillator.The tuning inductance 48 is tapped as indicated at 52 to provide aconnection to ground for the input grid through a grid re-V sistor 53,the cathode circuit 54 being likewise connected to ground. The resistor53 is bypassed by a capacitor 55. A coupling capacitor 56 is alsoconnected between the low potential side of the circuit 46 'and thecathode circuit and ground.

Anode current is supplied to the tube 45 throughthe tuning inductance58, an R. F. choke coil 68, and an anode voltage dropping resistor 6|from the supply lead 22. The tank circuit 41 is by-passed at its lowpotential side to cathode and ground through a suitable by-passcapacitor 62. The screen grid circuit indicated at 64 is connected withthe output end of the potential dropping resistor 6| through anisolating resistor 65, provided with a by-pass capacitor 66 foregilfiecting filtering of the potential on the screen With the foregoingarrangement the output frequency of the oscillator is eifectiveiydoubled tiplied also by the stage 5, resulting in an output from atransmitter in which this system is used of a virtually constantamplitude frequencymodulated carrier wave.

The frequency doubling and amplitude-modulation-eliminating amplifierstage 5 is represenvthrough a by-pass capacitor -circuit 88 is likewiseby-passed to cathode tative of any desired number of such stages whichmay follow the oscillator circuit in cascade connection, such as thesucceeding amplifier stage 6 which is also of the class C type. Thiscomprises an arnplier tube 10 having an input grid 1| coupled across aninput grid resistor 12 and through a coupling capacitor high potentialor output side of the tuned anode circuit 41 of the preceding amplifierstage 5.

The amplifier 18 is provided with a suitable tuned output circuit 14,comprising a tuned inductance 15 shunted by a variable tuning capacitor16 in the anode circuit thereof. Anode current is supplied from thesupply lead 22 and the terminal 23 through a supply circuit indicated at11 and a suitable R. F. choke coil 18 interposed between the lead 11 andthe low potential side of the tuning inductance 15. The anode circuit isby-passed to ground and cathode 19. The screen grid through a suitableby-pass capacitor 8| and is supplied with operating current through apotential dropping and filtering resistor 82 from the supply lead 11.The bias for the grid 1I is obtained from a source ||5.

Thel final amplifier stage may be of any suitable type and in thepresent example comprises the usual output amplifier tubes indicated at85 andv 86, connected in push-pull relation between a tuned inputcircuit 81 and a tuned output circuit 88. Anode and other operatingpotential may be obtained from any suitable source indicated by supplyterminals 89 and 99. In the present example, a battery 9| is connectedwith terminals 89 and 98 as a source of anode circuit. In a practicalembodiment of the invention which was found to be effective, theelements of tubes 85 and 86 were embodied in a commercial type tube 832with an operating potential of 450 volts supplied at the terminals 89and 98.

The output circuit 88 is coupled through a coupling coil or othersuitable means indicated at with ground 96 and the radiator or antennareferred to at 8, while the input circuit is coupled to the last stageof the multiplying amplifier through any suitable means, such as a linkcircuit or transmission line 91 having an input coupling coil 98 coupledwith the tuning inductance 15 and an output coupling winding 99 coupledwith the tuning inductance |80 of the input A circuit 81 for the finalstage amplifier. The grid circuits in this case, are biased from sourceH6.

From the foregoing description, it will be seen a radio transmittersystem in accordance with the invention comprises a self-excitedoscillator,v means for amplitude modulating the self-excited oscillatorwith a low percentage modulation by varying the anode or plate voltagethereof, such means including a modulation input connection between amodulation choke coil in the anode circuit of the oscillator and a tapon the tuned @Scillator circuit. The system further comprises 13 withthe one or more multiplier stages of the class C type. whereby theamplitude modulation component of the generated signal is removed. Inthe present example with the audio-frequency output of the modulatorsuperimposed upon the oscillator anode voltage, the percentagemodulation is limited to a low value by controlling the output voltageof the modulator, such as by adjustment of the gain control device 32.

Since the oscillator is thus amplitude modulated at a low percentagemodulation and being of a self-excited type, the variation in anodevoltage in the oscillator tube I produces a variation in frequency inthe oscillator circuit. That is, frequency modulation of the oscillatoris developed by the audio frequency output of the modulator. The outputof the oscillator is supplied to the amplifier stage which functions toincrease the amplitude of the voltage from the oscillator, to multiplythe frequency of the oscillator output signal, and to suppress theamplitude modulation component. This is because the depth of modulationis maintained at a low value, and at the same time the frequencydeviation of the frequency modulation component is multiplied by thesucceeding amplifier stages. The resulting output from the final stageamplifier and the radiator 8 is a substantially constant amplitudefrequency modulated carrier wave.

In designing any specific equipment using the present method ofmodulation, the depth of the amplitude modulation, the oscillatorfrequency, and the oscillator L/C ratio are selected so that suilicientgrid drive power is available between modulation peaks whereby themultiplier stages will increase the frequency deviation to a requiredamount at the desired output frequency. For this purpose, any number` ofadditional stages represented by the stage 8 may be added tothe stage 5.

During tests to determine the operating elliciency of the systemdescribed, it was found that frequency modulation was accomplished withpractically no resultant amplitude modulation and that the transmissionwith frequency modulation resulted in communication equally as good inevery respect as that resulting from amplitude modulation of the finalstage amplifier. Furthermore, frequency modulation signals from a systemsubstantially as described and embodying the invention, is received bestwith a broad intermediate frequency channel indicating that thefrequency deviation was wider than normal amplitude modulationsidebands.

Furthermore, with the system described, no critical adjustment isrequired for any of the components other than adjustment of themodulator output for low percentage modulation. Because of thesimplicity of the circuit employed and the relatively few number ofcomponent parts involved, a battery power transmitter may be provided ina relatively small space adapted for portable use. For example, thecircuit of Figure 1 may be embodied in a portable transmitter having aform substantially as shown in Figure 2, to which attention is newdirected along with Figure 1.

The component elements 3, 4, 5, 6, and 1 may be located in sectionalunits |02, |03, |04, |05, and |06, respectively, of Figure 2. Each ofthe units consists of a suitable shielded casing which may be joined andconnected to form a single transmitter unit with the antenna or radiator|01 extending verticallyand supported by insulated rear bracket means|08 and |09. The

microphone or other speech input device is 1ocated at the top of theunit as indicated at H0. The battery power may be provided by suitableelements carried within handle members indicated at III and H2 on eachside of the unit, and connected therewith by bracket means H3, asindicated. By providing the battery power in the handles of the devicethe individual battery units may be made accessible for inspection andrepair, and effects a reduction in the overall size of the main housingof the unit. Furthermore, by providing the device in sections, any onemay be removed for replacement and repaired more readily than if theywere provided in a unitary case, as a single instrument.

Thus far the invention has been shown and described in one of itspresent preferred forms both as to circuit and las to structure. Otherembodiments thereof mrv be made without departing from the principles,spirit and scope of the invention as defined in the claims.

I claim:

l. A radio transmitter system comprising in combination, a self-excitedoscillator including an oscillator tube having a cathode, an anode and acontrol grid, a tuning inductance connected between the anode tap onsaid inductance, circuit means for applying controlled anode current tosaid tube through said tap, means connected with said tap forsuperimposing amplitude modulation signals on said anode current. anamplifier biased beyond cutoff coupled with said tuning inductance forreceiving the modulated signal output from said oscillator and fordeleting the amplitude modulation component of said signal, and meansfor controlling the degree of amplitude modulation applied to saidoscillator, whereby effective frequency modulation within predeterminedoperational limits may be provided.

2. The combination with a self-excited oscillator comprising anoscillator tube and a tunable oscillator tank circuit therefor of means.for supplying anode current to said oscillator through said tankcircuit, means for applying amplitude modulation signals to said anodecurrent whereby the oscillator is frequency-modulated, means forlimiting the depth of amplitude modulation applied to said oscillator toa low degree, and an amplifier biased beyond cutoff coupled to saidoscillator to receive the modulated signal output therefrom and todelate the amplitude modulation component of said signal wherebyeffective frequency modulation within predetermined operational limitsmay be provided.

3. The combination with a self-excited oscillator comprising anoscillator tube and a tunable oscillator tank circuit therefor of meansfor supplying anode current to said oscillator through said tankcircuit, means for applying amplitude modulation signals to said anodecurrent whereby the oscillator is frequency-modulated, means forlimiting the depth of amplitude modulation 4applied to said oscillatorto a low degree, and an amplifier biased beyond cutoff coupled to saidoscillator to receive the modulated signal output therefrom and todelete the amplitude modula tion component of said signal, and meansconnected with said last-named amplifier for multiplying the outputfrequency of the signal de= livered by said oscillator, whereby thefrequency deviation range of the resultant frequency modulated signal isincreased and available for transmission, the degree of said amplitudemodulation being low enough to provide sufficient voltage to and controlgrid thereof, a.

drive the grid of said amplifier between modulation peaks.

4. A frequency modulation system comprising audio-frequency modulatormeans, a self-excited oscillator having an anode circuit coupled to saidmodulator means for receiving the modulation signal output thereof,means for controlling said modulation signal output to cause lowpercentage amplitude modulation or said oscillator, means for effectingvariation in oscillatorfrequency in response to variations in themodulation signal, and an amplifier biased beyond cutoff coupled to saidoscillator to suppress the amplitude modulation component of theoscillator signal output and provide a final signal for transmission atpredetermined frequency, said percentage being low enough to providesufficient voltage to drive the grid of said amplier between modulationpeaks.

5. A frequency modulation system comprising audio frequency modulatormeans, a self-excited oscillator having an anode circuit coupled to saidmodulator means for receiving the modulation signal output thereof,means for limiting and controlling said modulation signal output tocause law percentage amplitude modulation of said oscillator, means fortuning and controlling said oscillator to cause effective Variation inthe frequency thereof in response to the modulation signal output toprovide effective frequency modulation signals, and a frequencymultiplying amplifier biased beyond cutoff coupled to said oscillator toincrease the effective frequency deviation range and to suppress theamplitude modulation component of the signal output and 36 provide aresultant signal of predetermined frequency for radiation, saidpercentage beinglow enough to provide suicient voltage to drive saidamplifier above cutoff between modulation peaks. 6. A frequencymodulation system comprising audio frequency modulator means, aself-excited oscillator having an anode circuit coupled to saidmodulator means for receiving the modulation signal output thereof,means for limiting and controlling said modulation signal output tocause low percentage amplitude modulation of said oscillator, means fortuning and controlling said oscillator to cause effective variation inthe frequency modulation signals, a frequency multiplying amplifierbiased beyond cutoff coupled to said oscillator to'lncrease theeffective frequency deviation range and to suppress the amplitudemodulation component of the signal output, an auxiliary amplifier biasedbeyond cutoff coupled to said first named amplifier for furthermultiplying the output signal frequency thereof to a predeterminedvalue, and means for further amplifying and radiating the resultantfrequenuy modulated output signal. from said auxiliary amplier, saidpercentage being low enough that each cycle of voltage derived from theoscillator is high enough to render said amplifiers conducting betweenmodulation peaks.

. JOHN C. GEIST.

REFERENCES CITED The following references are of record in the Name DateDome July 4, 1933 Number

